[WebAssembly] Handle END_LOOP in unreachable BB in CFGStackify

Summary:
This fixes crashes when a BB in which an END_LOOP is to be placed is
unreachable and does not have any predecessors. Fixes PR41307.

Reviewers: dschuff

Subscribers: yurydelendik, sbc100, jgravelle-google, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60004

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357303 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

@@ -411,7 +411,9 @@ void WebAssemblyCFGStackify::placeLoopMarker(MachineBasicBlock &MBB) {
   // Mark the end of the loop (using arbitrary debug location that branched to
   // the loop end as its location).
   InsertPos = getEarliestInsertPos(AfterLoop, BeforeSet, AfterSet);
-  DebugLoc EndDL = (*AfterLoop->pred_rbegin())->findBranchDebugLoc();
+  DebugLoc EndDL = AfterLoop->pred_empty()
+                       ? DebugLoc()
+                       : (*AfterLoop->pred_rbegin())->findBranchDebugLoc();
   MachineInstr *End =
       BuildMI(*AfterLoop, InsertPos, EndDL, TII.get(WebAssembly::END_LOOP));
   registerScope(Begin, End);